[usbdev] Add ASSERT_KNOWN on outputs
[usbdev] Add ASSERT_KNOWN on outputs

This is required for D1 signoff.

Signed-off-by: Tom Roberts <f3b1a53ee1cbeb35a47033dea932030093853292@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan